Optical Raman amplifiers are particularly attractive for use in optical communications networks for their broad wavelength range. In wavelength division multiplexed (WDM) networks, this is particularly important. The Raman gain spectrum is broadened by providing pump energy at a plurality of different wavelengths. In typical Raman amplifiers, channel monitors are provided to monitor the individual channel gain across the transmission spectrum. Information from the channel monitor is provided to a controller to regulate the pump power of the plurality of pump sources at different wavelengths.
A Raman pumped fiber amplifier with a constant pump level will not produce a well-controlled output signal in response to large variations in the input signal level. When the input power suddenly increases due to the addition of new channels, the Raman pump is depleted, which causes the output power per channel at the end of the pumped transmission fiber to decrease more than desired. When the input power suddenly decreases because channels have been dropped and the Raman pump level is not lowered accordingly, the Raman gain becomes too high and the output power per channel at the end of the pumped transmission fiber increases more than desired. A channel monitor provides gain information which identifies which pump source power to regulate.
Providing a channel monitor for each Raman stage is quite costly in both equipment and maintenance. It is desired to reduce the cost and complexity of such systems by eliminating the need for channel monitors at every stage. By simplifying the pump control algorithm, the pump control can also be significantly accelerated.
Accordingly, a simplified method for automatic dynamic gain control in optical Raman amplifiers remains highly desirable.